This invention relates to the field of battery chargers, more particularly those chargers using current mode pulse-width modulated (PWM) control systems having a variable duty cycle to control output voltage and charging current.
The sensing of output current in order to both limit it to some maximum desired value and to adjust the output voltage are known in the prior art. The use of output voltage information to cause current limit foldback is also well known. Such techniques have been used in linear power supply design, and were accomplished by conductive connection to the output terminals.
Unlike linear practice where an isolating output line transformer is used, in the design of switched-mode type supplies the regulating element is ordinarily conductively connected to the power input mains. In switching practice, the regulating element, specifically the switch, is often electrically isolated from the output.
Some switcher art has taught the isolation of the switcher control circuits from the actual switcher element such as by using gate driving transformers and then connecting all of the switcher circuitry to the output side. This makes it possible to use control signals directly electrically connected to the output terminals to provide regulation, voltage adjustment, and current limiting and foldback functions. However, with such an approach it is much more difficult to power up (bootstrap) because there is no power on the output side to start the switcher when the circuit is first turned on. Circuits must be added to supply startup power to the output side.
It has been discovered that various switching parameters present at the switcher or input (the primary side of the isolation transformer) side can be used to control the output in a desired manner. In particular, instantaneous output current can be determined from instantaneous primary current. Similarly, average output current can be determined from time-average-integrated instantaneous primary current.
To completely charge a lead acid battery to the maximum possible charge without damaging the battery requires 14.2 volts (for a nominal 12 volt battery). Appliances and light bulbs designed for use on lead-acid battery powered systems, such as vehicles, are typically designed for voltages from 12 to 13.6. Higher voltages seriously shorten the life of such light bulbs and other items. Therefore, it is desirable that a power converter designed for vehicular light bulbs and appliances produce a voltage of 14.2 under light loads such as an ampere or two, thus assuring that a battery can be fully charged overnight, yet adjust its output voltage to 13.6 (or less) volts under heavier loads, thus providing power at a proper voltage to lights and other appliances, as well as charging the lead acid battery to just below a full charge, ready for final "trickle charging." It is to be understood that under ideal conditions the charger will provide more than 13.6 volts only when no load other than the battery is on the system.
It is desirable to sense the current to cause this adjustment in the primary of a switching converter, because the secondary currents reach levels such as 50 amperes and sensing such high currents is uneconomical of both components and power. Further, resistive sensing is preferred for minimum cost and smallest size. The voltage adjustment must, however, be made at the secondary side of the isolation transformer, because the voltage comparison for the regulating feedback loop takes place at the secondary and is preferably coupled to the primary through an opto-isolator.
Under short circuit output conditions the pulse width becomes very small (a very short "ON" time) in order to keep the current from rising above the desired current limit. Under short circuit, no power is being delivered to the load, so the only power required to maintain the current at limit is to overcome internal losses. In the embodiment disclosed, the parasitic resistances are very low, which is another way of saying the circuit is very efficient, so a narrow pulse will provide sufficient energy to maintain current at the limit value.
It has been discovered that in the case of short circuit, the narrow pulse width by definition requires a very small duty cycle, typically less than 10 percent ON time and over 90 percent OFF time. This causes the output current, which will be the short circuit limit current, to flow in rectifying diodes for about 10 per cent of the time, and to flow in free-wheeling diodes for about 90 per cent of the time. This places the free-wheeling diodes under a heavy stress.
This heavy stress is avoided by another aspect of the present invention wherein the current is reduced to a level well below the full-load or maximum value under conditions of short circuit.